This invention relates generally to semiconductor materials and, more particularly, to a method for growing direct-gap GeSn epilayers and nanostructures directly on silicon substrates.
Light-emitting semiconductor devices rely on materials that possess direct band gaps. Interestingly, none of the elemental group-IV materials are direct-gap semiconductors: diamond, silicon, and germanium have indirect band gaps, and cubic α-tin is a zero-gap semi-metal. Compounds based on these elements, such as SiC and the Si1-xGex alloys, are also indirect-gap semiconductors. It has been recognized that the Si1-xGex system is a nearly ideal semiconductor alloy, with a lattice constant and interband optical transition energies that are essentially linear functions of x. See O. Madelung, Semiconductors—basic data (Springer, Berlin, New York, 1996). Because these semiconductors have indirect band gaps, however, the have been precluded from use as active layers in light-emitting diodes and lasers.
It has also been recognized, on theoretical grounds, that the group-IV SnxGe1-x system is a possible exception to the indirect band gap behavior of group IV materials. The band gap of the SnxGe1-x alloy is expected to undergo an indirect-to-direct transition, since the direct band gap has a value of 0.81 eV in Ge and becomes negative (−0.4 eV) in gray (α-) Sn. See M. L. Cohen and J. R. Chelikowsky, Electronic Structure and Optical Properties of Semiconductors (Springer, Heidelberg, Berlin, New York, 1989). A linear interpolation between Ge and α-Sn places the crossover at x=0.2, and this simple estimate agrees remarkably well with detailed electronic structure calculations within the virtual crystal approximation. See D. W. Jenkins and J. D. Dow, Phys. Rev. B 36, 7994 (1987); K. A. Mader, A. Baldereschi, and H. von Kanel, Solid State Commun. 69, 1123 (1989).
This knowledge has stimulated intense experimental efforts to grow SnxGe1-x compounds that are of high enough quality to be used for microelectronic and optical device applications. These efforts, however, have previously been hampered for a number of reasons. There is an enormous lattice mismatch (15%) between Ge and α-Sn, and the cubic α-Sn structure is unstable above 13° C. As a result, the system is highly metastable and cannot be produced in bulk form. Efforts have been made to grow metastable films of SnxGe1-x by molecular-beam epitaxy (MBE). See G. He and H. A. Atwater, Phys. Rev. Lett. 79, 1937 (1997); O. Gurdal, R. Desjardins, J. R. A. Carlsson, N. Taylor, H. H. Radamson, J.-E. Sundgren, and J. E. Greene, J. Appl. Phys. 83, 162 (1998); M. T. Asom, E. A. Fitzgerald, A. R. Kortan, B. Spear, and L. C. Kimerling, Appl. Phys. Lett. 55, 578 (1989). A major problem encountered in the MBE approach, however, is the low thermal stability of the materials and the propensity of Sn to segregate toward the film surface.
Some progress has been made, as described by H. Höchst, M. A. Engelhardt, and D. W. Niles, SPIE Procedings 1106, 165 (1989) and C. A. Hoffman, et al., Phys. Rev. B 40, 11693 (1989), but the large compositional dependence of the lattice constant limits this approach to a narrow range of compositions near the Sn-rich end. For the Ge-rich Ge1-xSnx alloys, which are of more interest technologically, pure Ge is an obvious choice as a substrate, and in fact fully strained SnnGem superlattices as well as random Ge1-xSnx alloys on Ge have been demonstrated. See W. Wegscheider, K. Eberl, U. Menczigar, and G. Abstreiter, Appl. Phys. Lett. 57, 875 (1990); O. Gurdal, et al., Appl. Phys. Lett. 67, 956 (1995). Unfortunately, a major disadvantage of Ge substrates is that tetragonally distorted Ge1-xSnx films on Ge are not expected to display an indirect-to-direct transition. Ge-rich SnxGe1-x films have been grown on Si substrates using Ge buffer layers. See P. R. Pukite, A. Harwit, and S. S. Iyer, Appl. Phys. Lett. 54, 2142 (1989); G. He and H. A. Atwater, Phys. Rev. Lett. 79, 1937 (1997). The optical properties of these MBE-grown films, however, differ very markedly from those observed in conventional semiconductor alloys: individual interband transitions are not observed, and the position of the band edges is obtained from fits that must incorporate transitions not found in pure Ge.
There is a need, therefore, for a method of growing direct gap, device-quality SnxGe1-x alloys directly on Si substrates without using buffer layers. It is an object of the present invention to provide such a method and semiconductor structure with a well defined Ge-like band structure.
It is another object of the present invention to such a method that is practical to implement and that can be used to produce such semiconductor structures in bulk device quality form.
Additional objects and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, on may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by the instrumentalities and combinations pointed out herein.